Mass memories are indispensable for storing and distributing information in the multimedia industry. The basic requirements for mass storage media and the corresponding recording and retrieving systems are 1) large capacity, 2) fast access, and 3) low cost. All these three requirements, in particular 1) and 3), are largely determined by the storage density i.e., the ability to store and recall large amounts of data in the smallest possible area or space. In most existing optical memories data are stored two-dimensionally (2D) across a planar surface with each bit of information occupying an area of a diameter approximately equal to the lateral size of the writing or reading laser spot; thus the only way to increase the recording density to reduce the spot size. Due to the diffraction nature of light, the smallest spot size that can be achieved by conventional optics is limited by the laser wavelength. To break this physical limit, several techniques are being investigated both theoretically and experimentally in the laboratory level, such as super-resolution and near-field optical recording. In either case, however, the data are still stored two-dimensionally; one will sooner or later face the physical limit problem again.
To overcome the restrictions imposed by present two-dimensional memories, various three-dimensional optical memories have been proposed. Among them are memories comprising a plurality of substrates separated by a light transmissive medium, two-photon memories, electron trapping memories, and holographic memories, for example. Since the information is stored in volume, three-dimensional optical memories have higher theoretical storage density than their planar counterparts. In addition, if the data can be arranged into two-dimensional pages and an entire two-dimensional page can be written or read in a single memory access operation, one can expect a very high data transfer bit rate and fast access.
For these three-dimensional optical memories, however, the maximum number of pages that can be stored is very dependent on the writing and reading techniques. In particular, for those which are based in the detection of intensity variation of reflected light, the maximum number of pages is generally low because each information layer requires a certain level of reflectivity in order to moderate reproduced signal level and modulation depth. Furthermore, crosstalk between adjacent layers is also inevitable unless the interlayer spacing is much larger than the depth of focus of the laser spot; this puts another limitation to the maximum number of pages that can be stored on a disk with a given thickness.
Holographic memory is, in principle, a true three-dimensional storage device. However, its performance at the present stage is limited by the lack of suitable materials. One of the distinguishing features of holographic storage is that both the amplitude and phase information of every single bit of original data are stored across the entire storage medium. Its page wise addressing capability allows very fast access speed and an extremely high data transfer rate. Although holographic memory may be viable in the future if suitable materials can be found, it is not a practical option.
In all these amplitude and/or phase-based techniques, the information layer basically can be considered as a spatial light modulator which modulates the laser beam either in the intensity, phase or polarisation. The information layer itself does not generate any photons or luminescence during the reproduction process. At present, the only method that is based on the detection of luminescence from the information layer is the so called electron trapping memory. Since each information layer is not required to have a certain level of reflectivity, this technique naturally allows storing large amounts of data three-dimensionally provided that a reliable page-by-page method can be found to address each information layer. To this end, the inventors have proposed in U.S. Pat. No. 5,502,706 a three-dimensional optical memory, comprising at least two thin film layers of different electron trapping materials for storing and releasing information in the form of light energy, in which each of the thin film layers of electron trapping material is sandwiched between a pair of insulating layers and a pair of transparent electrodes. By using such a structure, the page wise accessing is achieved through selectively supplying a voltage to the transparent electrodes to enhance either the storage or releasing of information to or from the addressed thin film layer.
A further explanation of this technique is given below with referring to FIGS. 1 and 2 which are reproduced from U.S. Pat. No. 5,502,706. The mechanism for information storage in electron trapping materials is firstly illustrated in FIG. 1. The most widely used host materials are wide bandgap alkaline earth compounds (e.g., SrS). The addition of rare earth atoms (e.g., Eu/Sm) into these compounds creates new energy levels associated with each atom within the forbidden band of the host materials. Taking SrS:Eu/Sm system as an example, during the write process short wavelength light excites electrons from the ground state of Eu to the corresponding excited state. The excited electron can either recombine to generate luminescence or hop to and be finally trapped by the Sm atoms. Since the electrons can remain in the ground state of Sm for a very long time, information is thus stored in these deep traps within the electron's lifetime. To reproduce the information, light with a longer wavelength is irradiated to the thin film to excite the electrons from the ground state of Sm atoms to the corresponding excited states; these re-excited electrons will transfer back to the excited states of Eu and finally return to the ground states of Eu through emission of photons. In this way, electron trapping materials can be used to store information in the form of trapped electrons; the information can be reproduced by detecting the number of photons emitted by the information layer. Further explanation of electron trapping is given in P. Goldsmith, J. Lindmayer, and C. Y. Wrigley, "Electron trapping: a new approach to rewriteable optical data storage", Proc. SPIE Vol. 1316, Optical Data Storage, 1990, pp. 312-320.
The charge transfer between the Eu and Sm atoms mentioned above is a very complicated quantum mechanical process and therefore the transfer efficiency is determined by many factors. To achieve a truly page wise addressable three-dimensional memory, one must find a means to control the transfer efficiency of each information layer separately from other adjacent layers. By utilising the phenomenon of electric field induced ionisation, the inventors of U.S. Pat. No. 5,502,706 have proposed a crosstalk reduction method of three-dimensional memory based on selective supply of voltage to the information layer to be addressed. The physics behind this technique shown in FIG. 2. The principle is that, if the host material is highly resistive and the inter-atom spacing is sufficiently large, then the only way to make the transfer possible from Eu to Sm or vice versa is to bring the electrons to the conduction band by the application of an external electric field, and then let them drift or diffuse to other impurity sites so that they can be recaptured.
The above mentioned approach, however, has disadvantages: (i) in order to reduce the transfer efficiency at zero electric field, the impurity spacing must be large, and the concentration will generally be 10 times lower than that used for two-dimensional memory as in the above examples; this significantly reduces the dynamic range of the readout signals; (ii) depending on the depth of the deep levels, to bring the electrons from the deep levels to the conduction band requires a very large electric field which is undesirable for practical application systems; (iii) the efficiency enhancement achieved as described in U.S. Pat. No. 5,502,706 is typically only about 1%, meaning that in practical terms the page selectivity is very low.